Clinically isolated syndromes (CIS) are isolated events of demyelination that occur within the central nervous system (CNS). It is estimated that between 30-70% of individuals who experience a CIS will go on to develop multiple sclerosis (MS), and that of all patients with MS, about 80% had a CIS as their onset episode. Clinical features of the CIS, such as whether it was monofocal or multifocal and number of clinically silent CNS lesions at the time of the CIS, do predict risk of later conversion to MS. However, little is understood about how environmental factors may also influence this progression. Vitamin D insufficiency, infection with Epstein-Barr virus (EBV), and cigarette smoking have all been identified as risk factors for developing multiple sclerosis (MS); the study proposed here will examine whether these factors also predict CIS conversion to MS and whether these factors influence early MS progression as measured clinically and by changes in CNS lesions. This study will include participants in three large randomized placebo controlled trials of the effect of various drugs on delaying or preventing conversion from CIS to MS: the Betaferon/Betaseron in Newly Emerging multiple sclerosis For Initial Treatment (BENEFIT) the Oral Cladribine in Early multiple sclerosis (ORACLE), and the Rebif Flexible Dosing in early multiple sclerosis (REFLEX). Collectively, these trials include over 1,600 participants who were diagnosed with CIS. All participants in these trials have provided multiple blood samples and have been followed for conversion to MS both clinically and by magnetic resonance imaging. Further, those who develop MS continue to be followed. Specifically, the aims of this study are to examine whether low serum 25-hydroxyvitamin D levels, elevated antibodies against EBV, or elevated serum cotinine levels (indicative of current smoking) increase the risk of conversion from CIS to MS and whether they increase progression, as measured by the Expanded Disability Status Scale, the MS Functional Composite, and changes in MRI parameters, early in the MS disease process. Time to event analysis and mixed effects modeling will be used in the statistical analysis. The wealth of information collected as a routine part of these trials provides an ideal opportunity to examine whether vitamin D levels, EBV antibody titers, or cigarette smoking status predict conversion from CIS to MS or influence progression early in the disease process. PUBLIC HEALTH RELEVANCE: Multiple sclerosis (MS) is a chronic progressive disease with a large negative impact on the quality of life of those affected, their families, and society. Individuals who experience a clinically isolated syndrome (CIS) are at an increased risk of developing MS. Whether current risk factors for MS, including insufficient vitamin D nutrition, infection with Epstein-Barr virus, or cigarette smoking, predict conversion from CIS to MS is not known. Understanding the role of these environmental factors in early MS progression may lead to new prevention and therapeutic strategies.